A pulsating brake pedal, which may be accompanied by a shuddering or jerky stop during normal braking, usually means a warped rotor or an out-of-round drum -- although it can sometimes be caused by loose wheel bearings, a bent axle shaft or loose brake parts. If the vehicle is equipped with ABS, however, some pedal feedback and noise is normal during panic stops or when braking on wet or slick surfaces. But you should not experience any ABS pedal feedback when braking normally on dry pavement.
The faces of a disc brake rotor must be parallel (within .0005 inch on most cars) and flat (no more than about .002 to .005 inches of runout) otherwise it will kick the brake pads in and out when the brakes are applied, producing a pulsation or vibration that can be felt in the brake pedal as the rotor alternately grabs and slips.
You can often see warpage in a brake rotor by simply looking at it. If the rotor has telltale glazed or discolored patches on its face, chances are it is warped. Measuring it with a dial indicator and checking it for flatness with a straight edge will confirm the diagnosis.
Resurfacing the rotor to restore the faces will usually eliminate the pulsation (unless the rotor is bent or is badly worn and has started to collapse in which case the rotor must be replaced). But it may only do so temporarily because of metallurgical changes that take place in the rotor. Hard spots often extend below the surface of the rotor. Resurfacing will restore the surface, but the hard spot may reappear again in a few thousand miles as the rotor wears. For this reason, GM and others recommend replacing warped rotors rather than resurfacing them.
Pedal pulsation caused by drum warpage isn't as common, but it can happen. A drum can sometimes be warped out-of-round by applying the parking brake when the brakes are hot. As the drum cools, the force of the shoes causes the drum to distort.
What causes a rotor to warp? Overtorquing or unevenly torquing the lug nuts with an impact wrench is a common cause. For this reason, most experts recommend using a torque wrench to tighten lug nuts when changing a wheel. There are also special torque-limiting extension sockets called "Torque Sticks" that can be safely used with an impact wrench to accurately tighten lug nuts. But a plain impact wrench should never be used for the final tightening of the lug nuts because most provide no control whatsoever over the amount of torque applied to the nuts.
Overheating can also cause rotors to warp. Overheating may be the result of severe abuse or dragging brakes. Defects in the rotor casting, such as thick and thin areas can also cause uneven cooling that leads to warpage. Hard spots in the metal due to casting impurities can be yet another cause.